1. Field of the Invention
The present invention relates to transistors and more particularly to monolithic transistor devices.
2. Description of the Prior Art
A circuit designer often has to make compromises when deciding which of the various transistor technologies available to select for a particular application. For example, in switching applications, bipolar junction transistors (hereinafter bipolar transistors or BJT's) are generally faster than metal-oxide semiconductor field-effect transistors (MOSFET's). However, BJT's are slower than MOSFET's for power transistor switching applications. A MOSFET can switch from the "on" state to the "off" state typically in less than 20 nanoseconds. The bipolar transistor may need longer than one microsecond to switch since it is usually driven deep into saturation in power applications.
Because of the relatively slow switching speeds of bipolar transistors, circuits utilizing them can experience higher switching losses when switched at high frequencies across high voltages. These switching losses occur as the transistor switches between its low voltage, high current "on" state and its high voltage, low current "off" state. During this transition, there is a finite period of time in which there is both a significant current flow in the device and a significant voltage drop across the device such that a substantial amount of power is dissipated. Generally, the slower the device, the larger the switching losses.
An additional disadvantage of a bipolar transistor is evident when it is connected to an inductive load. The bipolar transistor often fails (i.e., continues to conduct) after it is attempted to turn it off due to the so-called "reverse-biased second breakdown" phenomenon. When the base of the transistor is reverse-biased in order to turn it off, the voltage across the device suddenly rises due to the inductive load. Since there are still stored carriers in the device during the turn-off period, it continues to conduct current for a few microseconds until all the stored carriers are removed. Thus, there is conduction at a high voltage, resulting in a very large instantaneous power dissipation. This reverse-biased second breakdown can occur at voltages below that which cause avalanche breakdowns.
MOSFET devices have their own disadvantages associated with them. The MOSFET generally has a significantly lower transconductance (gm) than the bipolar transistors of similar chip size. That is, for a given change in the input voltage, a greater change in the output current is produced in the bipolar transistor than in the MOSFET. As an example, a base drive voltage of only two volts can typically drive a bipolar transistor deep into saturation, whereas in order to produce a similarly high current in a MOSFET, an input voltage of from six to eight volts is often required. Furthermore, the MOSFET generally has a higher "on resistance" (RON) than a bipolar of similar chip size. As a consequence, the MOSFET will typically dissipate a greater amount of power at a given current than the bipolar transistor.
Thus, it is seen that both bipolar and MOSFET devices have their own particular advantages and disadvantages, particularly with respect to power applications. Hence, a designer selecting a transistor for one characteristic, such as switching speed, will incur disadvantages in other characteristics such as "on resistace" and transconductance.
It is an object of the present invention to provide a monolithically merged semiconductor device obviating, for practical purposes, the above-mentioned limitations of prior devices.